1. Field of the Invention
The invention relates generally to semiconductor structures. More particularly, the invention relates to semiconductor structures comprising semiconductor substrates with multiple crystallographic orientations.
2. Description of the Related Art
As semiconductor technology has advanced and semiconductor device dimensions have decreased, various effects merit additional consideration when fabricating semiconductor structures. Charge carrier effects are of particular importance since they often influence semiconductor device operating parameters such as off currents, on currents, drive currents, saturation voltages, switching voltages and the like.
Recent attention has focused on the role of semiconductor substrate crystallographic orientation upon semiconductor device performance. Different crystallographic orientations typically have different physical and electrical properties, such as charge carrier densities and piezoresistance coefficients. In light of those physical and electrical properties, a trend has evolved that involves selection of specific semiconductor substrate crystallographic orientations in order to favor or optimize specific electrical performance within individual semiconductor devices.
As an example, Nobel et al., in U.S. Pat. No. 6,580,154, teaches a method and a resulting structure that provide semiconductor devices located lateral to a (110) silicon semiconductor substrate crystallographic orientation plane to effect enhanced conduction in a <110> direction. The enhanced conduction is realized within the context of hole charge carrier mobility. In addition, Guarini et al., in U.S. Pat. No. 6,830,962 teaches a method for fabricating a semiconductor substrate with multiple crystallographic orientations. The method utilizes a semiconductor-on-insulator substrate having top and bottom semiconductor layers of different crystallographic orientation. The method further utilizes a selective surface etch process, an epitaxial growth process and a separation by implantation of oxygen (SIMOX) process to provide the semiconductor substrate with multiple crystallographic orientations. Finally, Yeo et al., in Pub. No. 2004/0195646 teaches a method for forming a silicon-on-insulator semiconductor substrate with different crystallographic orientations. The method utilizes recrystallization of an amorphous silicon layer.
Desirable are additional methods and structures that allow multiple semiconductor structures and devices to be located upon multiple crystallographic orientations of a single semiconductor substrate.